Within the art of flexible pipe there are two main groups, bonded pipes and unbonded pipes. An example of a bonded pipe is disclosed in WO 02/061317. Bonded pipes usually consist of a number of reinforcement layers bonded together with bonding agents to affect a strong bond therebetween.
Flexible pipes of the present type are of the unbonded type, which differs substantially from the bonded type. Flexible unbonded pipes are well known in the art in particular for offshore transportation of fluids and are for example described in API (American Petroleum Institute) 17B. “Recommended Practice for Flexible Pipe”, which is used as a standard within the field. Flexible unbonded pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armoring layers of metal, such as steel on the outer side of the inner liner (outer armoring layer(s)). The flexible pipe usually comprises an outer sheath, provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armor layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
In order to have sufficient strength, in particular to prevent the collapse of the inner sealing sheath, the flexible pipe may for certain applications comprise an armor layer located inside the space defined by the inner sealing sheath. Such inner armoring layer or layers are normally referred to as a carcass.
In this text the term “unbonded” means that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known unbonded, flexible pipe normally comprises at least two armoring layers located outside the inner sealing sheath, which armoring layers are not bonded to each other neither directly nor indirectly via other layers along the pipe. The pipe layers can therefore move with respect to each other, and thereby the pipe becomes bendable not merely for rolling up for transportation but also sufficiently flexible for being applied in a dynamic application where a high flexibility is often important for being able to withstand the high impacts high mechanical and turbulent forces it may be subjected to in use. If the flexible pipe does not have the required flexibility it may often be damaged e.g. by being subjected to undesired deformations and/or breaking of reinforcement wires.
The above-mentioned type of unbonded, flexible pipes is in particular useful for dynamic offshore applications for the transport of fluids including liquids and/or gasses, in particular petrochemical products. In particular such unbonded, flexible pipes may be used for risers, but they may also with advantage be used in other applications e.g. for flow lines. A riser is a pipe which at least in a length section is used for transporting a fluid vertically i.e. closer to or away from the seabed. Unbonded, flexible pipes in the form of risers may extend partly or totally from one sea surface installation (an installation on or near the surface of the sea) to another, from one sea surface installation to a sub sea installation or from the seabed up to a sea surface installation.
Risers need to be able to move dynamically and to withstand the forces and dynamic motions they will subjected to such as high or varying water pressures along the longitudinal axis of the pipe and dynamic motions due to turbulence, wind, high and varying waves, water current and other weather conditions which set the sea in motion.
The unbonded, flexible pipe should have a high flexibility to follow the motions it will be subjected to during use. If the armoring layers—which are usually of wires e.g. helically wound wires—are too stiff, the wires may be deformed or even break and the whole pipe will be damaged. On the other hand the motion of the pipe and/or the sea surface installation may result in an overbending of the pipe, which is equally damaging.
The object of the invention is to provide an unbonded, flexible pipe which has a high flexibility and simultaneously a low risk of overbending even when subjected to dynamic applications.